Battery Management Systems (“BMS”) are typically used for the control of charging and discharging of a battery or batteries in a battery power source system. The BMS typically manages the operation of such systems but can also provide important fail safety actions to prevent potentially hazardous consequences should a fault occur.
A BMS is important for applications using a battery made up of long strings of cells based on intercalation chemistry (“intercalating cells”). The batteries (sometimes called “battery packs”) typically consist of series strings of large individual cells or alternatively strings of “modules”, each module comprising a batch of small cells connected in parallel. Any references hereinafter to “cell” should be read, where the context permits, to include such modules.
Batteries consisting of intercalating cells including without limitation all lithium-ion and lithium polymer cell types are commonly used. These batteries are used in various applications, including for electric vehicles (“EV”), in some aircraft, for grid storage and for emergency or stand-by power.
A typical BMS measures the voltage of each cell in the battery and typically has the following discrete functions:                a) It stops charge when the first cell reaches its maximum voltage;        b) It stops discharge when any cell reaches a minimum voltage;        c) It performs “active balancing”, that is equalisation of cell voltages on each charge/discharge cycle (see below);        d) It identifies voltage fluctuations symptomatic of a failed or failing cell and raises an exception.        
The day-to-day purpose of the BMS is primarily to ensure that, over each charge/discharge cycle, all the cells in a battery are charged and then discharged as fully as possible, while ensuring there is no over-charging or over-discharge of any of the cells. Monitoring for signs of failing cells is a very important secondary function.
These functions are needed because:                a) Over-charging of any cell can cause overheating and even fire.        b) Over-discharge of one or more cells in a battery can result in accelerated ageing or can cause “voltage reversal” where the more fully charged cells in effect charge the weaker cells backwards, typically resulting in irreversible damage to the affected cells.        c) Where the voltage monitoring apparatus of an active-balancing BMS identifies a defective or failing cell, it can signal an alert to trigger a suitable safety response before the cell overheats or catches fire.        
Active balancing (equalisation of cell voltages under load on each charge/discharge cycle) is a method intended to ensure that all cells remain at a consistent state of charge (SoC) throughout the battery life. The rationale for this is that it is thought necessary to prevent the SoC of individual cells in a battery drifting apart over a series of charge/discharge cycles. Voltage drift is a well-known phenomenon observed when testing long strings of various cell types under various conditions. In any situation where SoC drift occurs, charging and discharging without active balancing progressively limits the amount of charge and discharge that can be achieved without causing some cells to be overcharged or others to be over-discharged. The effect is to progressively reduce the usable capacity of the battery.
An active-balancing BMS uses one of a variety of techniques to try to equalise charge between cells. This may involve shunting charge between cells or draining (“dumping”) charge from any cells which are approaching over-charge. This process usually occurs towards the end of the charge cycle and is generally referred to as “top-balancing”.
The purpose of top balancing is to try to ensure that, on termination of charge, all cells are equally and fully charged. No cell is permitted to be charged to a voltage higher than a predetermined safe maximum. All cells are thereby intended to reach full discharge more or less equally, so maximising the usable capacity of the battery on each charge cycle.
However, there are three key drawbacks with using an active-balancing BMS:                a) It requires many electrical connections which results in many possible points of failure        b) Cell voltage monitoring is not a fully adequate means of assessing the state of health (“SoH”) of individual cells        c) Top balancing uses the measured voltage of the cells as part of the technique for achieving full battery charge. However, the voltage of intercalating cells may not stabilise for a period of time after charging ceases and very large transitory divergences in cell voltages can occur in well-balanced batteries at the top of charge. Research into lithium cells has shown that cell voltage is not a good indication of SoC unless the cells are open circuit and “rested” i.e. sufficient time has passed since the last current flow in the cell for the cell voltages to stabilise. This can only be done statically either before assembly or during routine maintenance when the cells can be allowed to rest. It follows that a technology based on balancing the battery at the top of charge may partly unbalance the cells, leading to a loss of usable battery capacity.        
The in-service record of active balancing BMS reflects these drawbacks. A significant number of fires in vehicles and other applications using active balancing BMS have been found on investigation to result from the very intense heat generated by thermal runaway of a lithium ion battery during charging. There have also been cases of fires starting in vehicle batteries while being discharged in active use. This is typically associated with loose cell connections; with operational currents during discharge typically up to several hundred amps, a loose or poor connection in a battery can quickly generate very high local temperatures.
Likewise, at least two instances of lithium battery fires have occurred in a modern commercial aircraft type even though duplicate active BMS were installed. The US National Transport Safety Board is currently reviewing the design of the battery system as part of its investigations.
Though less catastrophic, cell damage caused by accidental over discharge remains a significant issue affecting battery life and capacity which current BMS have not eliminated.
Partly because of the risks and uncertainties inherent in current BMS techniques, BMS used in commercially manufactured EVs and other applications often only use a proportion of the capacity of the battery. One current production EV for example allows the use of only about 65% of the available cell capacity, the remainder providing a safety margin to avoid the risk of overcharging or undercharging. This is significant given the high cost, bulk and weight of batteries and in particular the challenge of maximising the range of an EV on a single charge.
Concern about fail-safety of BMS has led to increased application of temperature monitoring of batteries and cells, designed to detect signs of thermal runaway before a fire results. While these methods have tackled some of the inherent safety weaknesses of the active balancing BMS, this is at the expense of added expense and complexity.
FIG. 1 is a schematic representation of a typical active balancing BMS based on cell voltage monitoring. For this illustration the primary consumer of power is assumed to be a motor 7 controlled by a motor controller 6.
The battery itself consists of a number of cells 2 joined by cell straps or bus-bars 3 connecting alternate positive terminals 4 and negative terminals 5. The battery management system master unit 1 has a dedicated sense wire 8 connected to each cell 2. The master unit controls the motor controller 6 and charger 9 which receives power from an external power source 10. The motor controller supplies power to the motor 7.
There are many variants of this topology using master-slave configurations or even cell-level BMS boards.
There are a number of issues with the BMS such as that shown in FIG. 1 which rely on detecting anomalies in the measured voltages on each cell to determine faults with the batteries as well as charging levels. Whilst the cell voltage can be used as an indicator of individual cell overcharge or over-discharge, it is not always completely reliable. In addition the voltage may not provide a clear indicator at an early stage of other problems. The main issues are:                a) For each cell there is at least one wired voltage-monitoring electrical connection 8, each being a possible point of failure. There may be a hundred or more of these wires in a typical EV battery of 300+ volts. If any one of the voltage sense wires shorts to ground the corresponding cell may be discharged to the point of damage;        b) The master unit 1 also needs to be constructed to receive and handle a large number of high voltage sense inputs from the above arrangement. This creates problems of reliability and maintenance in the master unit itself;        c) When a cell is damaged or is failing this may not immediately affect the voltage on the cell concerned and so there may be some delay before a voltage deviation sufficient to alert the BMS occurs. This may be too late to prevent significant damage or even catastrophic failure of the battery.        d) If a cell strap 3 is loose or has a poor connection, this can lead to significant resistance, causing the voltage readings to be incorrect with consequent under or over-charging of the cell;        e) If one of the voltage sense wires goes open circuit, the voltage reading will likewise be incorrect;        f) Poor connections between the cells such as on the inter-cell straps or bus bars or internal cell failure can lead to rapid overheating and potential damage. These may not initially show as voltage anomalies;        g) There is no mechanism to detect short circuits caused for example by foreign objects coming into contact with the external connections of the battery since this may appear as a load on the battery but not as a significant individual cell voltage deviation.        h) If the master unit 1 itself, or the connection between the master unit and the charger, fails for any reason, the charger may not receive instructions to shut down at the end of charge. This is the probable cause of some of the incidents cited earlier.        
In addition to the above, the use of top balancing using cell voltage monitoring has another potential drawback.
This arises from the fact that the individual cells in a battery will not all have exactly the same capacity. Therefore even if top balancing could be performed perfectly, the cells with slightly lesser capacity would be fully discharged before those with greater capacity. In other words, balancing the cells at the top of charge inevitably means that they are unbalanced at the bottom. If discharge continues past the point where the weakest cell is exhausted (i.e. has discharged to the point below which further discharge is undesirable), that cell will continue to receive current reducing the voltage across it. Driving a cell beyond this point can lead to irreversible damage to the cell. Further forced discharge can continue until the cell voltage is pushed into voltage reversal which in practice always destroys the cell.
One of the other functions of a typical current generation BMS is to shut down discharge when any individual cell voltage gets too low. Such a BMS should therefore prevent this by shutting down discharge before cell damage occurs, but if a system failure occurs, loss of one or more cells is highly likely. Additionally in some cases a small residual drain from the battery continues after shut down i.e. when the BMS is inactive. This can result in destruction of the pack: again there have been published incidents where an EV with a near discharged battery has been parked for a long period. The small residual current exhausts the battery and (being unbalanced at the bottom) some cells are driven into voltage reversal.
Damage to cells due to this kind of voltage reversal is, for this reason, fairly common with top balancing BMS. In contrast, bottom balanced battery packs are considered less likely to be damaged in this way and are thus more able to survive very deep accidental over-discharge of the battery without loss of cells. This is because although the cells are all over-discharged, as they are balanced none are pushed into voltage reversal because no cells are significantly stronger or weaker than any other at the bottom of discharge.
In summary, there are a number of issues with existing BMS arrangements. The present invention sets out to overcome or ameliorate at least some of these issues.
Experience with current batteries managed with a conventional BMS is that the individual cells do not maintain their SoC relative to each other without an active balancing stage as part of the charge cycle. This divergence in the SoC of cells over time is generally believed to be caused by a number of factors. However, it is also thought to be difficult or impossible to avoid and the solution generally applied is to use active balancing to maintain a relatively consistent SoC for all cells.
It is, however, possible to remove the routine causes of loss of balance in normal use.
One significant factor in the divergence of SoC of cells is the presence of unbalanced parasitic loads on the cells in a standard BMS configuration. These parasitic loads can be caused by wiring in low voltage instrumentation or ancillary apparatus to one cell or a group of cells, but less obviously it is possible to have this effect through the arrangement of the voltage monitoring connections themselves. Accepted practice in most domains is to regard the current used by a voltmeter, typically of the order of 20-40 micro-amps, as negligible. Even though a voltage measuring circuit may only draw a tiny current, however, this drain will be continuous throughout much of the operating life of the system. Over time, it can be shown that this current drain may amount to a significant amount of asymmetric charge removal.
A typical voltage monitoring architecture in a battery of 100 cells will involve measuring the difference between the pack negative and each cell junction. Individual cell voltage will be computed by subtraction. In such a structure, the current draw from the first cell might be around 35 micro-amps, flowing through that cell only. The current draw from the second cell will also be 35 micro-amps, but will flow through the second and first cells. This continues for each additional cell such that unbalanced load will cumulate, with the difference in over 100 cells in this example amounting to some 3.5 milliamps. A voltage monitoring device left permanently connected for 12 months (8,760 hours) would unbalance the pack (first cell compared to hundredth cell) by around 30 amp hours. Given that a typical battery of this size might have a total capacity of say 70-80 amp hours, a loss of balance on this scale would be very material. More sophisticated voltage monitoring systems with isolated monitors on each cell reduce the severity of the problem but do not eliminate it.
A second significant source of cell imbalance in batteries is that internal impedance and other factors can cause temporary voltage divergences between cells for reasons other than differing SoC. This effect appears to be at its worst when the battery is nearly full, close to the end of the charging process. Unfortunately, this is typically the point at which a conventional BMS attempts to balance the pack.
In a conventional BMS, top balancing is generally preferred, i.e. balancing when the battery is close to full charge, compared to bottom balancing (equalising cell voltages at or near full discharge). One significant reason for this is because bottom balancing must be carried out close to the point of complete discharge of the battery and this may not be convenient. Batteries are not always fully discharged when they are required to be recharged. For example in an EV, the battery may be only partly discharged when the vehicle is no longer required but needs to be fully recharged ready for future use. If the battery is only partly discharged, then to carry out bottom balancing, it would be necessary to complete the discharge process. This will take time which might be required for charging as well as wasting energy.
As noted above, since the top of charge is the point in the discharge cycle where cell voltages tend to diverge for reasons other than state of charge, a BMS that balances voltages at this point may actually introduce a degree of imbalance.
A third factor causing cell imbalance is a process of self-discharge which can occur in defective cells as a consequence of internal “soft shorts”. Soft shorts are thought to be caused by impurities of defects in manufacture (see for example “Advanced Mitigating Measures for the Cell Internal Short Risk” by Darcy et. al 2010), or sometimes as a result of abuse of the cells. Even though this discharge may not be very large, it would over time cause those cells to go out of balance compared to the others.
The usual procedure for charging a single lithium ion cell is to use a constant current/constant voltage (CC/CV) algorithm. This normally involves three steps (possibly more if the initial cell voltage is very low) which can be seen in FIG. 7. The steps are:                a) The battery is charged at a constant pre-determined current 52 (which will depend upon the size of the cell and other factors). During this phase the cell voltage 51 will gradually rise. This is known as the “Constant Current” or “CC” phase.        b) When the cell voltage reaches a predetermined level the charger will start reducing the current to keep the cell voltage at a constant level. This is known as the “Constant Voltage” or “CV” phase.        c) When the current drops to a second preselected level (typically 5% of the original charge current), charging is positively terminated        
This approach is often described as the CC/CV charge profile. The voltage at which the charger changes from the CC to CV phase will depend on the cell and is typically defined by the manufacturer as a charging upper limit voltage.
The same CC/CV profile is frequently used for charging batteries composed of multiple cells in series, and here it is less successful. Even if the overall battery voltage is kept constant in the CV phase, the individual cell voltages may diverge widely. A typical conventional BMS may have to command early charge termination if the voltage of one or more cells rise excessively, even if none of the cells are fully charged.